1. FIELD OF THE INVENTION
This invention relates to mill tooth rotary cone rock bits.
More particularly, this invention relates to milled tooth rotary cone rock bits with hardfacing material metallurgically bonded to the cutting edges of the teeth.
2. DESCRIPTION OF THE PRIOR ART
It is known to hardface steel teeth milled into rotary cones to enhance the cutting action of the teeth and to inhibit erosion and fracture of the teeth as the milled toothed bit works in an earthen rock formation.
U.S. Pat. No. 4,836,307 entitled Hard Facing For Milled Tooth Rock Bits assigned to the same assignee as the present invention describes a hardfacing to reduce erosion and abrasion associated with drilling in earthen formations.
The hardfacing for teeth on a milled tooth rock bit comprises at least 65% by weight of a mixture of tungsten carbide particles and a balance of steel bonding the carbide particles together and to the cutter cone of the rock bit. The tungsten carbide particle mixture comprises from 35% to 80%, and preferably from 65% to 80%, by weight 20 to 30 mesh cemented tungsten carbide, and from 20% to 65%, and preferably from 20% to 35% by weight 40 to 89 mesh single crystal mono-tungsten carbide.
Experience has shown however that, where hardfacing is applied around sharp corners retention of the hardfacing at the corners is difficult. For example, a milled tooth formed in a chisel shape is hardfaced over the apex from the side or flank of a tooth, across the crest of the tooth and down the opposite flank, the thickness of the material is less around the sharp corners. The hardfacing being thin in the corners tends to wear rapidly and flake off exposing the relatively softer steel forming the base of the tooth.
The present invention provides a means to retain the hardfacing material on the milled teeth especially around the vulnerable corners of the tooth adjacent the chisel type crest of the tooth.
In addition, the edges formed at the flank and end faces of each milled tooth is rounded to enhance retention of the hardmetal to the tooth.
Moreover, the present invention also provides a means to enhance the durability of the chisel crest of the milled tooth between the opposite radiused corners of the tooth.